The present invention relates to a line tracking control method for enabling a robot to cooperate with a workpiece moving on a conveying device such as a conveyor.
Manual operations on production lines are being replaced with industrial robots in recent years. The manual operation can perform a cooperative job matching a particular workpiece on a production line based on the sophisticated abilities of a human being to collect and process information. However, various technical measures should be taken in controlling a robot which is employed as a substitute for a human being. For example, it has been difficult to control the hand of a robot so as to attend appropriately a workpiece moving on a conveying device such as a conveyor. Generally, where an industrial robot is used on a production line, the robot starts moving in a tracking direction at the time a tracking start signal is applied to the robot, and the robot moves at the same speed as that of the conveying device while working on the workpiece on the conveying device.
The robot starts moving in the tracking direction when supplied with the tracking start signal as shown in FIG. 4 of the accompanying drawings. At this time, the conveying device has already reached a certain speed Vc in the tracking direction as illustrated in FIG. 3. Since the robot starts from the zero speed, it is delayed with respect to the speed of the conveyor by a time constant .tau. indicated as a gradient. Theoretically, the delay is not eliminated unless the time constant is zero. For those robots which have large inertia, the delay cannot be neglected since the time constant is large. This situation is explained in FIG. 5 in which designated at TP is a conveying device such as a conveyor, RB an industrial robot, WP a workpiece, lc a commanded distance, and Tl a delay that the robot suffers in following the workpiece. As is apparent from FIG. 5, when the robot RB is supplied with a tracking start signal as the workpiece WP passes thereby, the robot RB starts moving in the tracking direction in order to grip the worpiece WP. However, since there is a delay Tl between the robot RB and the workpiece WP, the robot RB is unable to grip the workpiece WP.
To compensate for the delay, there has been proposed a system for applying, to a robot, a control signal which includes a variable added to the rate of movement of the conveyor, the variable being commensurate with the robot delay.
FIGS. 6 and 7 are diagrams illustrative of such a system. Denoted at t is a time, Vr is a robot speed, CV a corrective variable for compensating for the robot delay with respect to the workpiece, the corrective variable being indicated as a shaded area.
Since the robot delay=(conveyor speed).times.(time constant)/2, the speed of the conveyor is monitored from time to time, and the corrective variable is varied from time to time to meet the conveyor speed. More specifically, the corrective variable is expressed by EQU d=(l/t).times..tau..times.(1/2) (1)
where t is the sampling period, l the distance of movement in the sampling period, and .tau. the time constant of the robot. If the corrective variable in the ith sampling period is smaller than that in the preceding (i-1)th sampling period, then the difference therebetween is added in i (ith sampling period). If the corrective variable in the ith sampling period is larger than that in the preceding sampling period, then the difference is subtracted. Thus, the ith commanded distance l for the robot is: EQU l=li+di-di-1 (2)
With the aforesaid system, the robot that starts moving from the stopped condition with respect to the conveying device that has already been moved can be controlled without a delay with respect to the conveying device.
Since, however, the above control system effects a delay compensation for every variation in the speed of the conveying device or conveyor, it has the following problem:
It is assumed that the conveyor speed varies slightly, and the distance of movement in the sampling period t (which is regarded as being constant for the sake of brevity) changes from l to l+.delta.. By putting the value l+.delta. in the equation (2), we get ##EQU1## This means that when an input from the conveyor is varied by .delta., then the command to the robot is varied by: ##EQU2##
Generally, .tau.&gt;t, and for a system in which the sampling period is short, i.e., which has good sensitivity, .tau./2t is increased. The conventional control system tends to amplify small variations in the speed of the conveyor. This is problematic since the robot is adversely affected such as by vibration.